C++ : Is Namespaces definition allowed before '#include <files>' .

Question

I am using a single namespace in multiple files and I have inserted "namespace abc {" at the start and "}" at the end of each of file through a script (except main). Therefore '#include " comes under the namespace in each file. When I compile, it doesn't work (not recognizing the system functions etc).

But if I define namespace after the '#include ' lines, everything works fine. what is the problem here?

Answer 1

The problem is that by putting the headers inside a namespace, you're making them declare functions in that namespace -- but the definitions (implementations) of those functions don't exist in that namespace, so when you link, they can't be found and linking fails.

To give a concrete example, let's say you had a header that declared a function int f(int). By enclosing that inside the braces for a namespace, you're turning that into a declaration for int somenamespace::f(int).

While int ::f(int) has been defined, int somenamespace::f(int) hasn't, so you can't link.

Note that this doesn't apply to extern "C" functions. They basically ignore namespaces, so (for example) something like:

namespace x {
    #include <stdio.h>
} 

won't affect the normal C functions in stdio.h.